Pneumatic rubber passenger and truck tires are composed of elements which conventionally include a tread of a rubber composition. The tread rubber is sometimes desirably compounded to provide a tire with a relatively low rolling resistance with reasonable wear and traction.
Although it may be desired to compound the tire's tread composition to reduce the rolling resistance of the tire without substantially reducing the tire's traction features, tire traction might be expected to be somewhat sacrificed as may be evidenced by its decrease in wet and dry skid resistance.
Various rubber compositions have been prepared for various purposes, some of which have included the tire treads. Often tire treads are composed of synthetic rubber or blends of synthetic rubber with natural rubber for the purpose of achieving desirable tire tread characteristics such as wear, traction and reduction in rolling resistance. Various synthetic rubbers have been used in the manufacture of tires with such treads including styrene/butadiene copolymers (prepared by emulsion or solution polymerization methods) sometimes referred to as SBR, high cis 1,4 polybutadiene rubber as well as high and medium vinyl (1,2-) polybutadiene rubbers. Sometimes a synthetic cis 1,4-polyisoprene may, at least in part, be substituted for the natural rubber in tire tread compositions.
Vinyl isoprene (3,4-polyisoprene) rubber has heretofore been taught to be useful for various purposes such as, for example, as a blend with other rubber in tire treads and use in industrial products such as vibration dampers, belts and shoe soles.
Representative of various patent and patent application disclosures are JP (Japan); 58-196,245; 59-96,143; 59-210,958; 62-104,847; 88-4,578; and 1-158,056; GE (Germany) 3,707,434; 3,720,461; 3,835,792 and P38-35-792.5; and U.S. Pat. No. 4,383,085 and 4,756,353.
Viscoelastics properties of a rubber, or a rubber blend, for a tire tread applications, are important. For example, a tan.delta property is the ratio of the viscous contribution to the elastic contribution for a viscoelastic rubber subjected to a dynamic deformation. Such property is typically represented in the form of a curve as a plot of tan.delta values versus temperature.
For a tire with a low rolling resistance, a tread rubber with tan.delta optimization for a temperature in the range of about 50.degree. C. to about 60.degree. C. is desired and a tan.delta optimization for a temperature range of about -20.degree. C. to about +10.degree. C. is desired for a tire with good wet skid resistance. It is difficult to adjust a rubber blend to achieve a tan.delta optimization substantially simultaneously for both temperature ranges and, thus, for both rolling resistance and wet skid resistance at the same time. Often, compromises have to be made.
By tan.delta optimization it is meant that the tan.delta value for the rubber, or rubber blend, is maximized in the region of approximately -20.degree. C. to about +10.degree. C. for a tire tread to have high wet skid resistance and tan.delta value is minimized in the region of about 60.degree. C. for a tire tread to have low rolling resistance.
Although various rubber compositions are taught to provide various benefits, some for tire treads, it continues to be desirable to provide a pneumatic tire having a rubber tread having an enhanced rolling resistance and/or treadwear commensurate with reasonable traction qualities.